docs: clarify install-from-source version stamp and Claude Code schema fallback

[carlosmoradev]

🔗 Linked Issue

Closes #67

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🏷️ PR Type

• type:docs — Documentation only

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📝 Summary

• Rewrites the version-stamp tip in docs/INSTALLATION.md (Windows and macOS/Linux) so it is no longer two consecutive commands that leave the user with two binaries and engram version still printing dev. The new tip presents two explicit, mutually exclusive options.
• Adds a focused troubleshooting callout in docs/AGENT-SETUP.md for the Claude Code marketplace schema error, pointing at claude update and naming Bare MCP as the fallback.

📂 Changes

File	Change
docs/INSTALLATION.md	Replaces the inline "Optional: build with version stamp" block in both Windows and macOS/Linux sections with a callout that shows Option 1 (go install -ldflags) and Option 2 (go build + move) as separate alternatives. Warns the reader not to run both.
docs/AGENT-SETUP.md	Adds an "If the marketplace command fails with a schema error" callout right under the marketplace command in the Claude Code section.

🧪 Test Plan

• Unit tests pass locally: go test ./... (no code changes; ran as a sanity check)
• E2E tests pass locally: go test -tags e2e ./internal/server/... (no code changes; ran as a sanity check)
• Manually verified that go install -ldflags="-X main.version=local-$(git describe --tags --always)" ./cmd/engram produces a binary whose engram version reports the stamped value instead of dev. The new instructions match observed behavior.

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✅ Contributor Checklist

• Linked an approved issue (Closes #67)
• Added exactly one type:* label (type:docs)
• Ran unit tests locally
• Ran e2e tests locally
• Docs updated (this PR is the doc update)
• Commit follows conventional commits format
• No Co-Authored-By trailers

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💬 Notes for Reviewers

• The current .claude-plugin/marketplace.json uses "source": "./plugin/claude-code", not "source": "git-subdir", so the exact failure mode the issue cites is not reproducible against main today. I kept the troubleshooting callout intentionally generic ("schema errors on some manifest fields") and named the Invalid schema: plugins.0.source: Invalid input text only as one example, so the guidance still helps users on older CLIs who hit any related schema issue (including any future change to the manifest).
• I deliberately did NOT invent a "minimum Claude Code version" number. The original issue admitted the exact minimum was not precisely known, so the callout points at claude update (which always upgrades to the latest) instead of a specific version that might be wrong.
• Both INSTALLATION.md blocks already had a one-line version-stamp tip. The rewrite preserves the same intent but resolves the PATH gotcha the issue described.

[carlosmoradev]

Heads up: I do not have permission to add labels on this repo, so type:docs is missing. Whenever a maintainer can add it, the "Check PR Has type:* Label" gate will pass.

[Alan-TheGentleman]

Reviewed the docs diff. The schema-error troubleshooting and the version-stamp clarification (B1 vs B2, avoiding the double binary on PATH) are accurate and match the actual go install behavior. Approving.