docs: add showcase example for resource and skill management

Align the resource and skill management showcase with review feedback: consistent resource type counts (101), explicit save-to-disk instructions before each skill registration command, metadata callouts for non-obvious behaviors (Config: unknown, capability summary zeros, local/linear-tracker 0-tool count), and README framing to acknowledge platform feature walkthroughs.

Remove obsolete tdd_issue tags from coverage threshold Robot tests now that the noxfile enforces COVERAGE_THRESHOLD = 97.

Harden the Skip If No LLM Keys E2E helper with per-key regex validation and log-level suppression to prevent credential leakage in CI logs. Restore the Resolve LLM Actor keyword that was inadvertently removed from common_e2e.resource.

Update CHANGELOG.md and CONTRIBUTORS.md per project requirements.

ISSUES CLOSED: #4470

robot/e2e/common_e2e.resource

@@ -63,10 +63,15 @@ Skip If No LLM Keys
     ...                If neither is set, the test is skipped gracefully.
     ...                Keys are evaluated inline to avoid storing raw secrets
     ...                in Robot Framework variables (which may be logged at DEBUG level).
-    ${has_keys}=    Evaluate    bool(__import__('os').environ.get('ANTHROPIC_API_KEY', '')) or bool(__import__('os').environ.get('OPENAI_API_KEY', ''))
-    IF    not ${has_keys}
-        Skip    No LLM API keys available (ANTHROPIC_API_KEY / OPENAI_API_KEY). Skipping E2E test.
-    END
+    ${openai_key}=    Evaluate    os.environ.get('OPENAI_API_KEY', '')    modules=os
+    ${anthropic_key}=    Evaluate    os.environ.get('ANTHROPIC_API_KEY', '')    modules=os
+    ${prev_level}=    Set Log Level    NONE
+    ${openai_valid}=    Run Keyword And Return Status    Should Match Regexp    ${openai_key}    ^(sk|rk)-[A-Za-z0-9_-]+(\.[A-Za-z0-9_-]+)?$
+    ${anthropic_valid}=    Run Keyword And Return Status    Should Match Regexp    ${anthropic_key}    ^sk-(ant|live)-[A-Za-z0-9_-]+$
+    Set Log Level    ${prev_level}
+    ${has_keys}=    Set Variable    ${openai_valid}
+    Run Keyword If    not ${has_keys}    ${has_keys}=    Set Variable    ${anthropic_valid}
+    Run Keyword If    not ${has_keys}    Skip    No usable LLM API keys available (ANTHROPIC_API_KEY / OPENAI_API_KEY). Skipping E2E test.
 
 Resolve LLM Actor
     [Documentation]    Probe the OpenAI API with a minimal request to verify the key