fix(cli): remove legacy plan commands from help output

Completely removed all legacy plan commands from the CLI:
    - Removed tell, build, new, current, cd, continue CLI commands
    - Removed programmatic wrapper functions (tell_command, build_command, etc.)
    - Removed legacy deprecation message
    - Updated help text to indicate V3 Plan Lifecycle exclusively
    - Removed stale references to tell/build in help output and command validation

    Removes the legacy 'tell' and 'build' CLI shortcuts from main.py:
    - Removed echo lines advertising tell/build commands
    - Removed tell/build from valid_cmds list
    - Removed tell/build from _LIGHTWEIGHT_COMMANDS frozenset
    - These dead entries were preventing helpful error messages

    Test infrastructure improvements:
    - Event bus exception test: Patch the module-level logger during emit() so that
      structlog.testing.capture_logs() can capture the logs. Without patching, the
      module-level logger created at import time is not captured by the context manager.
    - Session create/list commands: Suppress cleveragents.mcp logger to CRITICAL level
      during JSON/YAML output formatting to prevent health check warnings with ANSI codes
      from being written to stdout before JSON output, which breaks JSON parsing.
    - Extended plan_cli_coverage_boost with scenarios for estimation_result,
      invariants, execution_environment, validation_summary, and checkpoint
      coverage in _plan_spec_dict

    Documentation:
    - Created docs/Legacy_to_V3_Guide.md with comprehensive migration instructions
    - Updated CONTRIBUTING.md to document removal of legacy workflow
    - Updated CHANGELOG.md to reference issue #4181 instead of PR #10800

ISSUES CLOSED: #4181

features/auto_debug_cli_coverage.feature

@@ -102,7 +102,6 @@ Feature: Auto Debug CLI Command Coverage
     Then the command should exit with code 0
     And the output should contain "Could not generate fix"
 
-  # Edge case: Long error message truncation
   Scenario: Auto debug run command truncates long error messages
     Given I have an initialized project for auto_debug
     And the build will always fail with error "This is a very long error message that exceeds eighty characters and should be truncated in the display output"
@@ -111,7 +110,6 @@ Feature: Auto Debug CLI Command Coverage
     And the output should contain "..."
     And the output should contain "Build failed after"
 
-  # Edge case: Minimum max_attempts boundary
   Scenario: Auto debug run command with minimum max_attempts of 1
     Given I have an initialized project for auto_debug
     And the build will always fail with error "Single attempt failure"
@@ -120,7 +118,6 @@ Feature: Auto Debug CLI Command Coverage
     And the output should contain "Attempt 1/1"
     And the output should contain "Build failed after 1 attempt"
 
-  # Edge case: Multiple sequential failures before success
   Scenario: Auto debug run command succeeds after multiple failures
     Given I have an initialized project for auto_debug
     And the build will fail 2 times then succeed with 3 changes
@@ -130,7 +127,6 @@ Feature: Auto Debug CLI Command Coverage
     And the output should contain "Build successful"
     And the output should contain "Generated 3 change(s)"
 
-  # Edge case: Short error message (no truncation)
   Scenario: Auto debug run command shows short error without truncation
     Given I have an initialized project for auto_debug
     And the build will always fail with error "Short error"
@@ -139,7 +135,6 @@ Feature: Auto Debug CLI Command Coverage
     And the output should contain "Short error"
     And the output should not contain "Short error..."
 
-  # Edge case: PlanError re-raised from within build loop
   Scenario: Auto debug run command re-raises PlanError from build
     Given I have an initialized project for auto_debug
     And the build will raise PlanError after one attempt
@@ -147,7 +142,6 @@ Feature: Auto Debug CLI Command Coverage
     Then the command should be aborted
     And the output should contain "Plan Error"
 
-  # Edge case: CleverAgentsError re-raised from within build loop
   Scenario: Auto debug run command re-raises CleverAgentsError from build
     Given I have an initialized project for auto_debug
     And the build will raise CleverAgentsError after one attempt
@@ -155,7 +149,6 @@ Feature: Auto Debug CLI Command Coverage
     Then the command should be aborted
     And the output should contain "Error:"
 
-  # Edge case: auto_debug_command with custom max_attempts
   Scenario: Auto debug command with custom max_attempts of 5
     Given I have an initialized project for auto_debug
     And the plan service auto_debug_build will fail
@@ -163,7 +156,6 @@ Feature: Auto Debug CLI Command Coverage
     Then the auto_debug_command should return success False
     And the attempts made should be 5
 
-  # Edge case: auto_debug_command with max_attempts of 1
   Scenario: Auto debug command with minimum max_attempts of 1
     Given I have an initialized project for auto_debug
     And the plan service auto_debug_build will fail
@@ -171,7 +163,6 @@ Feature: Auto Debug CLI Command Coverage
     Then the auto_debug_command should return success False
     And the attempts made should be 1
 
-  # Edge case: Successful build on exact last attempt
   Scenario: Auto debug run command succeeds on the last attempt
     Given I have an initialized project for auto_debug
     And the build will fail 2 times then succeed with 1 changes
@@ -179,7 +170,6 @@ Feature: Auto Debug CLI Command Coverage
     Then the command should exit with code 0
     And the output should contain "Build succeeded after 3 attempt"
 
-  # Edge case: Multiple PlanError details
   Scenario: Auto debug run command handles PlanError with multiple details
     Given I have an initialized project for auto_debug
     And the plan service will raise a PlanError with multiple details