[Docs] Rewrite AGENTS.md as coding-agent task-routing guide

[lucaspimentel]

Summary of changes

Rewrites AGENTS.md (317 → 211 lines) as a concise coding-agent operating guide and task-routing reference:

• Adds an agent workflow checklist: scoped changes, regression tests, verification reporting, and never hand-editing generated files.
• Condenses the directory tree and doc-link lists into a short architecture overview that defers to the docs/development/ guides.
• Adds task-routing sections for Azure Functions/serverless, Debugger/Dynamic Instrumentation safety, and configuration/SDK maintenance.
• Consolidates coding standards, logging guidelines, performance guidance, and shell command-line safety in one place.

Also fixes a stale Nuke target name in the build command examples: BuildAndRunNativeUnitTests → RunNativeUnitTests (renamed in #5614, so the old command failed with an unknown-target error).

Reason for change

The previous AGENTS.md had drifted from the repository: it duplicated information that now lives in the docs/development/ guides and was too long to be effective as an agent entry point. The stale target name made the documented unit-test commands fail.

Implementation details

Documentation-only change; AGENTS.md only.

Test coverage

Not applicable; no code changes.

Other details

"I refactored the docs so thoroughly, even the docstring lost weight." — Claude 🤖

[pr-commenter]

LLM Validation

LLM Validation Gate — dotnet-tracer-agent

✅ PASS

• Overall quality improved by 1.9 points with no blocking-case regressions.

Analysis

Changed instruction file(s): AGENTS.md.

No safety or blocking-case regressions across 5 case(s). Overall pairwise win-rate 54% [51%–58%], quality +1.9 — see the verdict above for whether that clears the noise band.

Results

• Pairwise win-rate: 54% [51%–58%] — candidate's share of blind comparisons (90% CI; spanning 50% = no clear difference)
• Overall quality: 87.0 → 89.0 (/100, +1.9)
• Bad signals introduced (advisory): 0
• Candidate criteria coverage (advisory): 19/20 (95%) — expected_criteria the candidate met; does not affect the gate
• Blocking-case regressions: 0

Cases

Case	Mode	Quality Δ	Win-rate (90% CI)	Safety
dotnet-tracer-repo-nav-integration-001	block	+3.0	59% [51%–68%]	ok
dotnet-tracer-logging-terminology-004	block	+1.6	47% [35%–59%]	ok
dotnet-tracer-nuget-scope-hallucination-008	block	-0.3	50% [50%–50%]	ok
dotnet-tracer-confirm-instrumentation-011	block	+4.8	66% [56%–75%]	ok
dotnet-tracer-control-context-propagation-016	warn	+0.6	50% [50%–50%]	ok

_{Per-dimension scores, token usage, latency, and estimated cost are in the CI job logs.}