docs(design): detail module contracts and sync interface map

[bouillipx]

Expand module design docs to a consistent detailed structure aligned with agent detailed docs and current implementation contracts.

Key changes:

• Reworked module READMEs to explicitly include public interfaces, core fields, and runtime flows.
• Added transport module main design entrypoint at docs/design/modules/transport/README.md.
• Appended explicit contract/field/flow snapshots to long-form context, hook, tool, and agent docs.
• Updated docs/design/modules/README.md index to reflect the new transport entrypoint and detail standard.
• Rewrote docs/design/Interfaces.md as a synced cross-module contract table derived from current kernel/interfaces/types.

Rationale:

• Reduce ambiguity between design docs and code signatures.
• Make cross-module integration points auditable and easier to review.
• Provide a single authoritative interface map for future implementation and refactor work.

Summary

• What does this PR change?
• Why is this change needed?

Scope (One PR One Thing)

• Primary objective:
• Out of scope:
• I confirm this PR handles a single objective only: [ ] Yes

Changed Files (required)

List all touched files and why each file changed.

File	Reason
path/to/file	reason

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Acceptance Criteria

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• Criteria 2

Test Evidence (required)

• Local commands run:
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• CI links or job names:
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• Evidence/output summary:
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If this PR changes high-risk runtime paths (auth/concurrency/execution control), include risk-matrix evidence.

Risk and Rollback

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Dependency and Lockfile Changes

• Lockfile changed in this PR: [ ] Yes [ ] No
• If yes, manifest updated in same PR (requirements.txt/pyproject.toml/package.json): [ ] Yes [ ] No [ ] N/A

Agent Rules Checklist (required)

Reference: docs/agent_rules.md

• Only task-related files changed; no opportunistic refactor
• Public interfaces/data structures unchanged unless explicitly required
• Tests added/updated and evidence attached
• Any skip/only/exclude usage is explained and reviewed
• Merge will be done by approved reviewer (no self-merge auto-ship)