RFC: Add repo/project context to sessions for cross-project filtering

[bdougie]

Summary

When using tapes across multiple projects, there's no way to identify which repo/project a session belongs to or filter sessions by project. This RFC proposes adding project context to the data model and surfacing it in the deck UI.

Problem

The deck currently shows all sessions in a flat list with no project association. Users working across multiple codebases have no way to filter or distinguish which sessions belong to which project.

Current Architecture

• Sessions are reconstructed at query time from leaf nodes in the DAG — not first-class entities
• The nodes table has no column for repo, project, or working directory
• The proxy captures LLM request/response pairs but no client context
• merkle.Bucket fields (Type, Role, Content, Model, Provider) are content-addressed — changes affect hashes

Proposed Approaches

Option A: Custom HTTP Header (X-Tapes-Project)

Clients send a header with each LLM request. The proxy extracts it and stores it as node metadata (outside the content hash).

Pros: backward-compatible, multiple projects in one DB, client-driven, doesn't break content addressing
Cons: requires client cooperation, each LLM client needs configuration

Option B: Per-Database Isolation

Leverage the existing dotdir package (local .tapes/ directories) so each project uses its own SQLite DB.

Pros: zero schema changes, natural isolation
Cons: no cross-project dashboard view, harder to compare across projects

Option C: Proxy-Side Detection

The proxy detects the git repo from its working directory or a config value.

Pros: no client changes needed
Cons: one proxy instance = one project, less flexible

Scope of Changes (Option A)

Layer	File(s)	Change
Schema	pkg/storage/ent/schema/node.go	Add optional project field
Proxy	proxy/proxy.go, proxy/worker/pool.go	Extract header, propagate to storage
Query	pkg/deck/query.go, pkg/deck/types.go	Add Project to SessionSummary and Filters
TUI	cmd/tapes/deck/tui.go	Add --project flag, filter tab
Web UI	web/deck/index.html, web/deck/deck.js	Add project dropdown and column
Web API	cmd/tapes/deck/web.go	Parse project query param

Open Questions

• Should this be a single approach or a hybrid (e.g., header + dotdir fallback)?
• How should existing sessions without project info be displayed? (e.g., "unknown" bucket)
• Should the project value be the git remote URL, repo name, or directory path?
• Should this also capture git branch context?

References

• pkg/deck/types.go — SessionSummary and Filters structs
• pkg/merkle/bucket.go — content-addressed fields
• pkg/dotdir/manager.go — local vs global .tapes/ directory support

[linear]

PCC-107

[bdougie]

We can do option C and keep B for the future. Option A is a big lift and likely unnecessary at this stage since we're local and not sharing across teams.